During certain phases of well drilling and development it is necessary to cut metal tubulars within the borehole, or to remove sections of downhole components such as packers. In order to achieve this, a cutting device must be lowered inside the tubular, then operated remotely to perform a cut.
One category of conventional tools for cutting tubulars are mechanical or hydraulic cutting or punch tools which are deployed on the end of drill pipe, coiled tubing or other tubular. Such devices suffer from the disadvantage of being cumbersome, as well as expensive to purchase, deploy and operate; the operation and deployment of the devices commonly requires a complete drill rig and several days to be completed. In situations where the tubular to be cut is narrow, devices in this category may be precluded.
Typically, devices in this category incorporate a number of large blades which gouge their way through the tubular. Gouging a cut through the tubular, i.e. forcing a punch through the tubular wall, rather than performing a precision cut, suffers from the disadvantage of requiring a large amount of energy. Typically, such cutting techniques leave the cut end of the tubular in a ragged condition, which can occlude subsequent operations involving the tubular.
Furthermore, the devices, which include a mechanism for anchoring the device within a tubular, typically utilize some form of hydraulic or pneumatic means for part of the deployment of that mechanism. The use of hydraulic and/or pneumatic means results in the devices requiring multiple cables/hoses which can lead to additional deployment problems when the device is to be used in a tubular, for example, a live oil well, having a seal and airlock mechanism and/or when a cut is to be made at great depth.
The positioning of the anchoring mechanism in relation to the cutting blade also affects the quality and accuracy of achievable cut. The tool can flex around the anchoring point, and the greater the distance between the anchoring point and the cutting blade, the greater the degree of flex and, accordingly, the greater the degree of inaccuracy in the cut.
However, besides inaccuracy in the cut, the major problem when the tool flexes is that as the blade is no longer cutting perpendicular to the tubular wall there is a considerable amount of rubbing on the side of the blade. This, combined with the vibration (caused by the lack of rigidity), results in a dramatic increase in failure rate.
In particular, as the cutting tip penetrates the wall of the tubular, the flexion acts like a spring, causing the tip to press outwardly (i.e. deeper into the tubular) and this causes the drive motor to stall and at the same time the cutting tip is destroyed. This is very common with overly long heads, and particularly because the tubulars are not always round, the tip may start cutting in one side before it makes contact on the whole tubular circumference.
Within traditional machining operations the control over surface speed and feed rate allows great variety in the material which can be cut; however, within known systems the feed rate of the cutter blade is often not controlled and is simply an output of the applied force or is mechanically linked to the rotational speed of the cutter blade. In both cases variation to the feed rate cannot be adjusted while the tool is in use. This lack of control can also account for considerable wasted time during a cutting operation as the cutting blade extension rate cannot be increased while the blade is not in contact with the tubular; likewise, as the cutting blade is returned into the tool body the feed rate again cannot be increased. It is estimated that in most cases the tool is only cutting for less than 50% of the time that the cutting head is being run. This has the negative effect of generating considerable heat within the electric motors and surrounding areas, which limits the life of the motors as in some cases the environmental temperature can be in excess of 200° C.